Internal olefins have hitherto been used or various applications, concretely a base oil for petroleum drilling oils, a raw material of detergents, a raw material of paper sizing agents, a base oil or raw material of lubricating oils, a raw material of chemical products, and the like. For example, though α-olefins having 14 carbon atoms are used as a base oil for petroleum drilling oils, when released in the environment the α-olefins having 14 carbon atoms exhibit fish toxicity, and therefore, in the case of a submarine oilfield, it is considered to be preferable to use α-olefins having 16 or more carbon atoms. However, since the α-olefins having 16 or more carbon atoms are inferior in fluidity to the α-olefins having 14 carbon atoms, it is necessary that they are internally isomerized to improve the fluidity.
In the case of performing an internal isomerization reaction of an α-olefin, it is important to prevent the formation of branched olefins. Similar to the problems in the field of surfactants, this is because the branched olefins are poor in biodegradability so that they remain in the environment over a long period of time. Then, there have been filed some patent applications regarding technologies for preventing a skeletal isomerization reaction so that only an internal isomerization reaction of an α-olefin is selectively performed. Though there is no definite index on how extent the skeletal isomerization should be prevented, for example, it is disclosed that a rate of skeletal isomerization reaction is controlled to less than 5% (see, for example, Patent Document 1).
In the internal isomerization reaction of an α-olefin using a zeolite catalyst, as a method for preventing the skeletal isomerization reaction, there are disclosed a method for using a pentasil zeolite catalyst containing from 1 to 10% by weight of nickel monoxide as a promoter (see, for example, Patent Document 1); and a method for using an alumino-phosphate-containing molecular sieve with one-dimensional pores having a diameter of from 3.8 to 5.0 angstroms as a catalyst (see, for example, Patent Document 2).
In the case of performing the internal isomerization reaction of an α-olefin, another important point resides in the matter that an oligomer is not formed in the reaction. When an oligomer increases, it is easily supposed that the fluidity of a formed internal olefin is deteriorated, and it is desired to suppress the formation of an oligomer as much as possible.
In claim 8 of Patent Document 1, it is described that a residual α-olefin is controlled to less than 5%, and in Example 3 thereof, it is actually proven that in isomerizing an α-olefin having 18 carbon atoms, the amount of formation of an olefin having 36 carbon atoms is controlled at not more than 4%.
In the light of the above, in order to improve the fluidity of an internal olefin, it is devised to reduce an oligomer at a 1% unit. However, it is still hard to say that the reduction of an oligomer is sufficient, and there may be the case where for the purpose of making the concentration of an oligomer low, the reaction product liquid must be further distilled.
Accordingly, a method for producing an internal olefin by stably isomerizing an α-olefin while preventing an oligomerization reaction is desired.
Patent Document 1: U.S. Pat. No. 6,054,629
Patent Document 2: U.S. Pat. No. 6,281,404